Subject: SIP Trunk outbound calls fail auth — provider requires SIP username in From header, not phone number
Hi all,
We’re connecting a German SIP trunk provider for outbound calls:
- Phone Number: +49xxxxxxxxxx (E.164)
- Termination URI: voip.provider.de
- SIP Trunk Username: e.g. K1234567X9 (an alphanumeric ID, not a phone number)
Outbound calls fail with: “max auth retry attempts reached for SIP invite”.
We contacted the provider’s support directly, and they confirmed the cause: our outbound INVITE signals the phone number in the From header’s user part. Their trunk requires the SIP Trunk Username to be signaled in the From header’s user part instead, for authentication to succeed.
We tried entering the SIP Trunk Username directly into the “Phone Number” field, but Retell rejects it with a validation error requiring E.164 format.
Has anyone run into this with a European provider that requires digest auth without a prior SIP REGISTER? Is there a way to configure the trunk so the From header’s user part uses the SIP Trunk Username instead of the phone number? Official support only sent generic onboarding resources so far, so hoping someone here has hit this before.
Thanks!